Impact resistant high strength steel

ABSTRACT

The present invention describes a novel martensitic steel with iron, nickel, boron, a carbide former, manganese, and carbon. The steel has substantially no cementite, substantially no interstitial carbon and substantially no interstitial nitrogen. There are ordered intermetallics dispersed in the iron and ordered intermetallics clustered at the dislocations. The present invention also describes a method of making high strength steel by alloying steel comprising iron and carbon with a strong carbide former, boron, and titanium, followed by heating the alloy steel to a sufficiently high temperature that the steel transitions to an austenitic, face centered cubic lattice phase and the strong carbide former removes substantially all of the carbon from the crystal lattice by forming a metal carbide other than iron carbide. The alloy steel is then quenched to a quench temperature with a quench faster than still air such that a body centered cubic lattice is formed by displacement, which forms ordered intermetallics in the alloy steel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 62/517,095 filed on Jun. 8, 2017, the contents of which are incorporated herein by reference in their entirety.

BACKGROUND

In order to decrease fuel consumption, increase safety and accelerate commercialization of aircraft, automobiles and other motor driven platforms, high strength materials have been developed with advantageous properties such as reduced weight and increased strength. However, many of these new materials have the disadvantage of high cost, making widespread usage unaffordable, thus restricting these materials to niche applications.

The steel industry has met this need with a general family of steels collectively known as Advanced High Strength Steels. These steels, including Interstitial free steels, Dual/Complex Phase, Transformation Induced Plasticity and martensite have been used in the automotive industry, however, they are reaching limits on specific mechanical properties and affordability. Therefore, there is a need for a new type of steel that has advantageous properties and reduced cost, thereby reducing weight, increasing safety, and maintaining or increasing affordability.

SUMMARY

The present invention provides a new type of Advanced High Strength Steel, or martensitic steel, comprising a) iron, at least some of the iron having dislocations, b) less than 10% nickel, c) between 0.0001 and 0.01% boron, d) more than 0.01% carbide former, e) less than 10% manganese, f) carbon, and g) less than 7% of all other elements. The steel has substantially no cementite, substantially no interstitial carbon and substantially no interstitial nitrogen. The steel also has ordered intermetallics dispersed in the iron and ordered intermetallics clustered at the dislocations.

The carbide former can comprise vanadium, titanium, niobium, zirconium, or a combination thereof.

In one aspect of the invention, steel can further contain titanium. The titanium in the steel can be more than 0.025% by weight. In another aspect, the titanium in the steel is more than 0.05% by weight. In another aspect, the titanium in the steel is more than 0.075% by weight. In another aspect of the invention, the titanium in the steel is less than 6.5% by weight.

In one aspect, the iron in the steel is at least 80% by weight. In one aspect, the nickel in the steel is less than 5% by weight. In one aspect, the manganese in the steel is less than 5% by weight. In one aspect, the carbon in the steel is at least 0.001% by weight. In one aspect, the carbon in the steel is at least 0.005% by weight. In another aspect, the carbon in the steel is between 0.005-0.2% by weight. In one aspect, the aluminum in the steel is at least 0.025% by weight, and the ordered intermetallics comprise ordered aluminum intermetallics. In another aspect, the titanium in the steel is at least 0.01% titanium, and the ordered intermetallics comprise ordered titanium intermetallics.

The present invention also includes a method of making a high strength steel comprising the steps of: a) alloying steel comprising iron and carbon with a strong carbide former, boron, and titanium; b) heating the alloy steel to a sufficiently high temperature that the steel transitions to an austenitic, face centered cubic lattice phase and the strong carbide former removes substantially all of the carbon from the crystal lattice by forming a metal carbide other than iron carbide; c) quenching the alloy steel to a quench temperature with a quench faster than still air such that a body centered cubic lattice is formed by displacement; and d) forming ordered intermetallics in the alloy steel. The invention also includes the steel made by the method.

In one aspect, the method step (d) comprises maintaining the alloy steel at a temperature between 200° C. and 750° C. for more than one minute. In another aspect, the method step (d) comprises heating the alloy steel. In another aspect, the method step (d) comprises having the alloy steel at the quench temperature or higher for more than one minute.

In one aspect, between method steps (b) and (c), the steel is formed by forging or hot rolling. In another aspect, the steel is cold rolled after method step c).

DRAWINGS

The detailed description of some embodiments of the invention is made below with reference to the accompanying FIGS. 1A-1B.

FIG. 1A is a schematic of steps involved in a method of making steel according to the present invention; and

FIG. 1B is a schematic of steps involved in the method of making steel according to the present invention.

DESCRIPTION

As used herein, the following terms and variations thereof have the meanings given below, unless a different meaning is clearly intended by the context in which such term is used.

The terms “a,” “an,” and “the” and similar referents used herein are to be construed to cover both the singular and the plural unless their usage in context indicates otherwise.

Definitions of chemical terms and general terms used throughout the specification are described in more detail herein, but unless otherwise indicated the chemical elements are identified in accordance with the Periodic Table of the Elements, CAS version, Handbook of Chemistry and Physics, 75th Ed., inside cover, or are terms that are well known to one of skill in the art.

Percentages referred to herein are percentages by weight.

As used in this disclosure, the term “comprise” and variations of the term, such as “comprising” and “comprises,” are not intended to exclude other additives, components, ingredients or steps.

As used herein, “dislocation” refers to a crystallographic defect or irregularity within a crystal structure of a material on an atomic scale. The presence of dislocations influences many of the properties of materials. Transmission electron microscopy (TEM) can be used to observe dislocations within the crystal structure of the material.

Interstitial free steel is well known in the steel industry where the higher cost of these types of steels are acceptable due to the excellent forming properties and a lack of yield point elongation. These steels typically have low yield strength, high plastic strain ratio, high strain rate sensitivity and good formability. Interstitial free steels have interstitial free body centered cubic ferrite matrix. Carbon is kept very low and is stabilized, or removed, from the crystal lattice structure of the steel, normally with titanium, niobium or both. The disadvantage of these steels is the relatively low strength of the ferrite.

Austenite steel has a face-centered cubic crystal structure. The carbon atoms lie in the interstices (holes) between the larger iron atoms. At slow cooling rates, the carbon moves ahead of the interface into the austenite iron by diffusion. Austenite steel may be strengthened by rapid cooling (quenching) by, for example, immersing the hot metal into liquid coolants such as water, oil, or liquid salts. Steels can also be quenched by air.

When austenite is rapidly cooled, a martensite phase is formed. Martensite, tempered martensite and bainite are high strength steels that are well known in the steel industry where very high strength is required. For the purpose of the present invention, martensite is defined as any steel or alloy steel that transitions from a face centered cubic lattice phase to a body centered cubic lattice phase predominantly by displacement and not by substitution.

In tempered martensite and bainite, the carbon migrates out of the lattice structure and forms a metal carbide. The disadvantage of these steels is the high carbon content required to form martensite. This causes a reduction in elongation and toughness with an inverse relationship to the increase in strength.

Additionally, Maraging and PH stainless steels are well known in the aerospace industry, where the higher cost of these types of steels are acceptable due to the excellent properties inherent in precipitating a nickel intermetallic. All current generation advanced high strength steels rely on carbon to some extent to add strength and/or facilitate other reactions. Maraging and PH stainless steels rely on formation of a martensitic steel matrix and, with aging, precipitation of a nickel intermetallic with aluminum, molybdenum, titanium, niobium, tantalum or other metals known to those skilled in the art. A typical Maraging steel may be composed of 18%-19% nickel, 8%-10% cobalt, 3%-5.5% molybdenum, 0.15%-1.6% titanium and 0.5% niobium, and after aging exceed 2,400 MPa yield and tensile strength. A typical PH stainless steel may be composed of 15% chrome, 4% nickel, 3% copper, and 0.15%-0.45% of tantalum and niobium. Carbon is kept low. This family of alloys will form martensite upon slow heat of the metal to a solution treatment temperature, and then let the metal air cool to room temperature, which forms intermetallics. The disadvantage of these steels is the high alloy content required to form a martensite steel matrix with slow cooling rates. The high alloy content also increases cost beyond what a high-volume application can justify.

Thus, there is a need for high strength interstitial free or predominantly interstitial free steel that is capable of forming shear martensite without high alloy additions and without carbon in the crystal lattice structure.

This invention provides a material that contains low alloy additions that remove carbon from the high temperature face centered crystal lattice and enable an industry standard quench to transform the steel to an interstitial free martensite without a temper treatment. When higher strength is required, the material of the invention may be heated to a precipitation hardening temperature to form an additional ordered intermetallic. The martensitic steel of the invention has substantially all of the carbon removed from the crystal lattice of the austenite prior to forming martensite, forming an interstitial free martensite that is strengthened not with carbon/carbon iron but by pinning the dislocations with an intermetallic.

In particular, the present invention provides a new type of martensitic steel, comprising a) iron, at least some of the iron having dislocations, b) less than 10% nickel, c) between 0.0001 and 0.01% boron, d) more than 0.01% carbide former, e) less than 10% manganese, f) carbon, and g) less than 7% of all other elements. The steel has substantially no cementite, substantially no interstitial carbon and substantially no interstitial nitrogen. The steel has ordered intermetallics dispersed in the iron and ordered intermetallics clustered at the dislocations.

The nickel and/or manganese may stabilize austenite and help to form martensite on cooling. It may also make the steel more impact resistant. Titanium and boron are very strong hardenability agents that work synergistically together to form martensite without the normal carbon additions that are used in plain carbon and alloy steel or the high alloy concentrations used in Maraging and PH stainless steels. Boron may also increase elongation and blunt crack propagation at grain boundaries and at the nickel intermetallic.

The carbide former used in the invention can be vanadium, titanium, niobium, zirconium, or a combination thereof.

In one aspect of the invention, the steel contains titanium. The titanium in the steel is more than 0.025% by weight. In another aspect, the titanium in the steel is more than 0.05% by weight. In another aspect, the titanium in the steel is more than 0.075% by weight. In another aspect of the invention, the titanium in the steel is less than 6.5% by weight.

In one aspect of the invention, the iron in the steel is at least 80% by weight. In one aspect, the nickel in the steel is less than 5% by weight. In one aspect, the manganese in the steel is less than 5% by weight. In one aspect, the carbon in the steel is at least 0.001% by weight. In another aspect, the carbon in the steel is at least 0.005% by weight. In another aspect, the carbon in the steel is between 0.005-0.2% by weight. In one aspect, the aluminum in the steel is at least 0.025% by weight, and the ordered intermetallics comprise ordered aluminum intermetallics. In another aspect, the titanium in the steel is at least 0.01% titanium, and the ordered intermetallics comprise ordered titanium intermetallics.

The present invention describes a method of making a high strength steel as shown in FIGS. 1A and 1B. First, as shown in FIG. 1A, the steel of the invention comprising iron and carbon is combined in, for example, a vessel (10) with a strong carbide former, boron, and titanium and poured (20) from the vessel (10) into an ingot or a slab (30). The constituents can be combined in any order or any way so that the result is the combination. Then the combination is heated to a sufficiently high temperature, such as, for example, 1200° C., such that the steel transitions to an austenitic, face centered cubic lattice phase (30) and the strong carbide former removes substantially all of the carbon from the crystal lattice by forming a metal carbide other than iron carbide. Depending on the application, a hot slab (30) can be rolled between rollers (40) to prepare a rolled hot slab (50), as shown in FIG. 1B. The rolled hot slab (50) of alloy steel is quenched to a quench temperature with a quench faster than still air such that a body centered cubic lattice is formed by displacement, and ordered intermetallics are formed in the alloy steel. The rolled hot slab (50) can be coiled (60) after it is rolled. A hot rolled slab (50) or coil (60) can be either maintained at temperature or heated to form additional ordered intermetallics. The invention also includes the steel made by the method.

Formation of ordered intermetallics in the alloy steel according to the present invention can be done in several ways. The first way is to keep the alloy steel at the same temperature as the temperature in which it was quenched. The second way is to raise the alloy steel above the temperature in which it was quenched. The third way is, after the quench, to allow the alloy steel to cool down, such as with, for example, air cooling, and then reheat, where the reheat temperature is more or less than the quench temperature.

In one aspect of the invention, the method comprises maintaining the alloy steel at a temperature between 200° C. and 750° C. for more than one minute after quenching. In another aspect, the method comprises heating the alloy steel after quenching. In another aspect, the method comprises having the alloy steel at the quench temperature or higher for more than one minute after quenching.

In one example, a low carbon alloy steel was made with 1.72% nickel, 0.38% titanium, 0.03% aluminum, 0.0022% boron, 0.021% carbon and 0.34% manganese. The steel was made by reheating the ingots to 1200° C., soaking in air for 1 hour per 25 mm of thickness, and then hot rolled with a finish rolling temperature of between 900° C. and 955° C.

After quenching from about 900° C. in water, the low carbon alloy steel of the invention formed an interstitial free martensite structure with a very high dislocation density. This steel is very tough, plastic or viscous-like compared to typical high carbon steels. Typical properties of the steel are:

Tensile Yield Total Elongation (TE) 820 MPa 680 MPa 12%

After cold rolling and aging at 510° C. for three hours, the nickel and titanium combined to form very hard, nanometer sized reinforcing rod-like structures with an average diameter of about 4 nm and length of about 15 nm that pin the dislocations and increase the strength of the steel. The titanium also formed additional intermetallic structures with the iron. Typical properties of the steel are:

Tensile Yield TE 990 MPa 980 MPa 7%

The strength, ductility and fracture toughness of the steel can be directly altered as required by the final application by reducing or increasing the amount of each of carbon, nickel, titanium, manganese, and other elements known to one of skill in the art, and changing the time and temperature of the heat treatment(s). The mechanical properties of the steel can also be altered by the order in which they are added to the steel.

In another example of the invention, an intermediate strength alloy steel was formed with manganese replacing nickel. For this, the material was made up of 1.48% manganese, 0.32% titanium, 0.033% aluminum, 0.0023% boron, 0.039% carbon with the balance iron and normal production and tramp elements.

In this example, the steel of the invention was made by reheating the ingots to 1200° C., soaking for 1 hour per 25 mm of thickness, and then hot rolled with a finish rolling temperature of between 900° C. and 955° C. The hot rolled coil was then quenched from about 900° C. in water to about 500° C. The ingot was then reheated to 500° C. and held for 24 hours to simulate a production hot rolling sequence. Average properties of the steel are:

Tensile Yield TE 854 MPa 765 MPa 16%

Other combinations of carbon, nickel, titanium, manganese, intermetallic forming metals and martensite stabilization elements may be used by those skilled in the art.

Although the present invention has been described in considerable detail with reference to certain preferred embodiments, other embodiments are possible. The steps disclosed for the present methods, for example, are not intended to be limiting nor are they intended to indicate that each step is necessarily essential to the method, but instead are exemplary steps only. Therefore, the scope of the appended claims should not be limited to the description of preferred embodiments contained in this disclosure. 

What is claimed is:
 1. A martensitic steel comprising: a) iron, at least some of the iron having dislocations, b) less than 10% nickel, c) between 0.0001 and 0.01% boron, d) more than 0.01% carbide former, e) less than 10% manganese, f) carbon, and g) less than 7% of all other elements, wherein the steel has substantially no cementite, substantially no interstitial carbon and substantially no interstitial nitrogen, and wherein there are ordered intermetallics dispersed in the iron and ordered intermetallics clustered at the dislocations.
 2. The steel of claim 1 wherein the carbide former comprises vanadium, titanium, niobium, zirconium, or a combination thereof.
 3. The steel of claim 2 wherein the carbide former is titanium.
 4. The steel of claim 1, further comprising more than 0.025% titanium.
 5. The steel of claim 1, further comprising more than 0.05% titanium.
 6. The steel of claim 1, further comprising more than 0.075% titanium.
 7. The steel of claim 1, further comprising less than 6.5% titanium.
 8. The steel of claim 1, comprising at least 80% iron.
 9. The steel of claim 1, comprising less than 5% nickel.
 10. The steel of claim 1, comprising less than 5% manganese.
 11. The steel of claim 1, comprising at least 0.001% of carbon.
 12. The steel of claim 1, comprising at least 0.005% of carbon.
 13. The steel of claim 1, comprising between 0.005 to 0.2% carbon.
 14. The steel of claim 1, wherein the steel comprises at least 0.025% aluminum, and wherein the ordered intermetallics comprise ordered aluminum intermetallics.
 15. The steel of claim 1, wherein the steel comprises at least 0.01% titanium, and wherein the ordered intermetallics comprise ordered titanium intermetallics.
 16. A method of making a high strength steel comprising the steps of: a. alloying steel comprising iron and carbon with a strong carbide former, boron, and titanium b. heating the alloy steel to a sufficiently high temperature that the steel transitions to an austenitic, face centered cubic lattice phase and the strong carbide former removes substantially all of the carbon from the crystal lattice by forming a metal carbide other than iron carbide; c. quenching the alloy steel to a quench temperature with a quench faster than still air such that a body centered cubic lattice is formed by displacement; and d. forming ordered intermetallics in the alloy steel.
 17. The method of claim 16 wherein step (d) comprises maintaining the alloy steel at a temperature between 200° C. and 750° C. for more than one minute.
 18. The method of claim 16 wherein step (d) comprises heating the alloy steel.
 19. The method of claim 16 wherein step (d) comprises having the alloy steel at the quench temperature or higher for more than one minute.
 20. The method of claim 16 wherein the carbide former comprises vanadium, titanium, niobium, zirconium, or a combination thereof.
 21. The method of claim 16 comprising between steps (b) and (c) forming the steel by forging or hot rolling;
 22. The method of claim 16, wherein the steel is cold rolled after step c).
 23. The steel made by the method of claim
 16. 24. The steel of claim 23 comprising at least 0.01% carbide former.
 25. The steel of claim 23 wherein the ordered intermetallics comprise ordered titanium intermetallics. 